1. Field of the Invention
The present invention pertains to a narrow divergence, single quantum well, separate confinement, AlGaAs laser.
2. Description of the Prior Art
Many attempts have been made by workers in the field to fabricate electronic image printers which utilize lasers to provide a light source. In particular, FIG. 1 shows the epitaxial growth structure of a commercially available laser, i.e., laser diode 10, which has been used in one such attempt to fabricate an electronic printer. As shown in FIG. 1, laser diode 10 is a single quantum well, separate confinement, AlGaAs laser (SQW-SC AlGaAs laser) which is comprised of the following sequentially-grown, epitaxial layers: (a) n-GaAs substrate layer 100; (b) n-AlGaAs cladding layer 101; (c) AlGaAs waveguide layer 102; (d) GaAs quantum well layer 103; (e) AlGaAs waveguide layer 104; (f) p-AlGaAs cladding layer 105; and (g) p-GaAs cap layer 106.
When a laser, like laser diode 10, is used in an electronic image printer, printer optics couples radiation output from laser diode 10 into a "writing spot" of radiation, which writing spot impinges upon a photosensitive print medium. In past attempts to fabricate a printer using a commercially available embodiment of laser diode 10, the writing spot produced has comprised only 40% to 50% of the radiation output from laser diode 10. This occurs because the printer optics is limited in its ability to collect radiation output from the commercially available embodiments of laser diode 10 since those diodes have a large far field angular divergence of the radiation output therefrom in a direction perpendicular to the diode junction. For example, the typical commercially available embodiments of laser diode 10 have a far field angular divergence of .about.60 degrees full width at half maximum (FWHM).
For this application the quantum well laser diode 10 is preferred to a double heterostructure (DH) laser for many well-known reasons. One such reason is that the quantum well laser has an advantage over a conventional three-dimensional DH laser in that the temperature sensitivity of its threshold current is lower for the quantum well laser than for the DH laser. The art predicts that this occurs because of the effect of dimensionality on the temperature dependence of the effective density of states. Specifically, in an ideal two-dimensional system, the density of states varies linearly as a function of temperature (T), rather than as T.sup.3/2 which occurs in a DH laser. However, as stated in an article entitled "Influence of the Barriers on the Temperature Dependence of Threshold Current in GaAs/AlGaAs Quantum Well Lasers" by P. Blood, E. D. Fletcher, K. Woodbridge, K. C. Heasman, and A. R. Adams in J. of Quantum Electronics, Vol. 25, No. 6, June 1989, pp. 1459-1467, at p. 1459, "From a theoretical standpoint, therefore, there is good reason to expect quantum well lasers in the GaAs/AlGaAs system to have a weak temperature dependence of threshold current. This has not been observed in practice: generally, the measured values of T.sub.o are below 250 K."
As one can readily appreciate from the above, there is a need in the art for a SQW-SC AlGaAs laser diode which has: (a) a relatively narrow far field angular divergence perpendicular to the diode junction and (b) a relatively weak temperature dependence of threshold current. In addition, there is a need for such a laser which also has a relatively high efficiency of radiation output Further, a laser diode which has the desired combination of relatively narrow far field angular divergence, weak temperature dependence of threshold current and relatively high efficiency could be used advantageously in an electronic printer.